What is the ruling on the zakat of land upon which an apartment building has been constructed for the purpose of selling residential units, knowing that the price of the land has increased significantly and that the proceeds from the sale of some units were spent on construction?

If land is purchased with the intention of personal use (al-qinya) and not with the intention of building on it and selling, then there is no zakat due on it. Zakat becomes obligatory only on the price of the apartments after one year has passed since receiving the payment. If the money was spent on construction and what remains does not reach the minimum threshold (nisab), then there is no zakat. However, if it was purchased with the intention of building and selling, then it is considered trade goods ('urud al-tijara), and zakat is obligatory on it by evaluating its market price each year after one year has passed since the original capital used to purchase the land. One-quarter of one-tenth (2.5%) is to be paid. As for installments, if you have other money in excess of your basic needs, no zakat is due on that excess money; it should be used to offset the debt, and then zakat is paid on the land. If you do not have money in excess of your basic needs, then the entire debt is deducted from the zakatable wealth. If what remains reaches the nisab, then zakat is due on it; otherwise, it is not.